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Species
"Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such group."
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When does speciation occur?
evolution altered physical or behavioral characteristics of populations long enough that they no longer interbreed
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How do new species arise?
- Branching evolution occurs when a single "parent" species diverges into two species
- -parent species continuing while second species arises from it
based on reductions in gene flow between populations of the same species
when gene flow is drastically reduced for a long period, populations will evolve separately
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Geographical Separation
most important factor in reducing gene flow between populations
- -from change in environment
- -or migration of part of population
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Allopatric Speciation
When geographic separation plays a part in the evolution of a new species.
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Extrinsic isolating mechanism
- Geographical separation:
- individuals of two populations cannot interbreed if they live in different places (the first step in allopatric speciation)
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What are the 6 intrinsic isolating mechanisms?
- Ecological isolation,
- temporal isolation,
- behavioral isolation,
- mechanical isolation,
- gametic isolation,
- hybrid invability or infertility
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Ecological Isolation
Even if they live in the same place, they can't mate if they don't come in contact with one another.
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Temporal Isolation
Even if they come in contact, they can't mate if they breed at different times
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Behavioral isolation
Even if they breed at the same time, they will not mate if they are not attracted to one another
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Mechanical isolation
Even if they attract one another, they cannot mate if they are not physically compatible
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Gametic Isolation
Even if they are physically compatible, an embryo will not form if the egg and sperm do not fuse properly.
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Hybrid inviability or infertility
even if fertilization occurs successfully, the offspring may not survive, or if it survives, may not reproduce (e.g., mule)
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Sympatric speciation
- Speciation occurs in absence of geographical separation
- (much rarer than allopatric speciation)
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Adaptive radiation
Speciation more likely when species introduced to an environment in which few other species of its kind exist
- such an environment has many niches available
- -leads to rapid specialization, a new which in turn leads to speciation.
rapid emergence of many species from a single species that has been introduced to a new environment
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The pace of speciation
Gradualist model: evolution proceeds at a slow, steady rate
Punctuated equilibrium model: organisms undergo long periods of stasis or little change, followed by relatively brief episodes of rapid speciation
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